1. Field of the Invention
The invention concerns an X-ray tube of the kind commonly used in radiology, in diagnosis by X-ray for example and particularly concerns means for reducing extra-focal radiation.
An X-ray tube comprises essentially two electrodes: a cathode and an anode contained in a glass sphere under vacuum and respectively fixed to the ends of this glass sphere. The cathode generally consists of a tungsten filament housed in a metallic part which is shaped appropriately so as act as an electronic lens and is called the focusing cap. The anode may consist of a cylindrical copper mass which bears, facing the filament, a plate made of a high-level X-ray emitting material, for example tungsten, in the case of a fixed anode tube. In the case of a rotating anode tube, the anode often consists of a massive disk, made of molybdenum or graphite for example, generally coated with tungsten. Of course, for special applications, other materials that those mentioned above may be used for the anode.
When the filament is made incandescent, and when a positive voltage of a few KV with respect to the cathode is applied to the anode, electrons emitted by the filament are accelerated towards the anode by the electrical field and bombard the anode or anti-cathode on a surface called the focal spot. The focal spot becomes the main source of X-ray emission. X-radiation is produced throughout the zone located in front of the anti-cathode except for grazing incidences.
The radiation yield of an X-ray tube depends on factors such as the current of electrons, the potential difference between the cathode and the anode and the atomic number of the material constituting the target on which the focal spot is formed.
In medical diagnosis, where the quality of a radiological exposure is of vital importance, the most important properties of the X-radiation source are those affecting the following two essential factors: sharpness and contrast.
Two types of fuzziness, having different causes, may affect sharpness:
geometrical fuzziness which results from the dimensions of the focal spot;
kinetic fuzziness due to the movement of the examined organ while the picture is being taken.
As regards factors of contrast, assuming moreover that the radiation quality and receiver are optimised so as to reveal those factors of contrast which depend on the source, the following may be cited:
the distribution in density of the electrons on the surface of the focal spot;
the place at which the electrons striking the anode outside the focal spot, called secondary electrons, fall or any other unwanted X-ray emission outside the focal spot. It must be pointed out that these unwanted sources of X-radiation are generally located on the anode itself, often near the focal spot, owing to the fact that electrons fall outside the focal spot, either coming from the cathode or bouncing off the focal spot, and create X-radiation called extra-focal radiation.
The geometrical fuzziness resulting from the dimensions of the focal spot affect the separating capacity, and the extra-focal radiation affects the attenuation of the contrast.
Studies on sharpness and contrast factors, such as those based on the known concept of modulation transfer function, where the investigation relates to the sinusoidal absorption image of an object, clearly show that the more extended the X-ray source, the more the image of an object is affected by a pepper-and-salt type of fuzziness due to the superimposition of a large number of images with points of low intensity as compared with the intensity of the focal spot.
2. Description of the Prior Art
In the prior art, it is sought to reduce these faults by the following two methods:
the first method, which can be applied especially to an anode with a massive target made of X-ray emitting material, consists in placing a collimator inside the casing of the tube, in the immediate vicinity of the surface on which the focal spot is formed. The implementation of this approach has great difficulties, especially as regards preserving proper electrical insulation between the anode and the cathode;
the second method can be applied especially to an anode with a basic structure consisting of a material that does not emit X-rays or emits them at a low level, for example materials with high atomic numbers. In this case it has been proposed to deposit on or include in the basic structure the X-ray emitting material solely at the place or places designed to be bombarded by the beam of incident electrons, namely, in the case of a rotating anode, solely on the focal track. One of the drawbacks of this arrangement is that the layer of X-ray emitting material, which constitutes the focal track, is deposited on a small width in order to restrict one dimension of the focal spot and, consequently, has a limited area: the layer thus deposited, which generally consists not only of a highly refractive material but also of a material which is a good heat conductor, has a very low volume so that, under the effect of electron bombardment, heat accumulates in this layer which becomes excessively heated and gets unstuck from the basic structure of the anode. Hence, no satisfactory device of this type is known to date.
The present invention relates to a fixed anode or rotating anode X-ray tube which can be used to obtain radiological images in which the sharpness and contrast are considerably improved. This is got by a new arrangement of the anode, which is easy to implement and which enables both a reduction in the extra-focal radiation and the limiting of at least one dimension of the focal spot while, at the same time, retaining those qualities of the anode provided by a massive structure of the X-ray emitting material. It is known by the delivered application DE-B-12 00 962 an anode provided with hollow parts to delimit the focal spot. Meanwhile these grooves constitute, by the fact that they are covered with thermo-emitting material, parasitic X-ray sources.